Utilizing the Transverse Thermoelectric Effect of Thin Films for Pulse Laser Detection

In this work, pulse laser detectors based on the transverse thermoelectric effect of YBa2Cu3O7-δ thin films on vicinal cut LaAlO3 (001) substrates have been fabricated. The anisotropic Seebeck coefficients between ab-plane (Sab) and c-axis (Sc) of thin films are utilized to generate the output voltage signal in such kind of detectors. Fast response has been determined in these sensors, including both the rise time and the decay time. Under the irradiation of pulse laser with the pulse duration of 5–7 ns, the output voltage of these detectors shows the rise time and the decay time of 6 and 42 ns, respectively, which are much smaller than those from other materials. The small rise time in YBa2Cu3O7-δ-based detectors may be due to its low resistivity. While the high thermal conductivity and the large contribution of electronic thermal conductivity to the thermal conductivity of YBa2Cu3O7-δ are thought to be responsible for the small decay time. In addition, these detectors show good response under the irradiation of pulse lasers with a repetition rate of 4 kHz, including the precise determinations of amplitude and time. These results may pave a simple and convenient approach to manufacture the pulse laser detectors with a fast response.


Introduction
Laser-irradiation sensors are of great value in detecting the laser parameters, such as power/energy, pulse duration, pulse frequency and shape. These measurements may be required as closed loop control arrangement or simple record-keeping. Nowadays, the commonly used laser-irradiation sensors are represented by three types according to their physical principles, such as photodiodes, pyroelectric and thermoelectric sensors [1].
The photodiode-based sensors detect lasers by converting photon energy into electronhole pairs in p-n junctions, giving rise to the fast response time of sub-nanoseconds. In addition, the sensitivity of this type of sensors is much higher, which allows the detectability of low power continuous wave (CW) and pulse lasers [1][2][3]. However, such a high sensitivity is usually companied with a relatively low power saturation threshold, which is not suitable for the detection of high power lasers. In addition, these photodiode-based sensors suffer from the limited spectral response, which is determined by the band gap of the semiconductors. For instance, sensors based on silicon possess a spectral response range between 0.2 and 2 µm [4].
Pyroelectric sensors transduce a temperature change into an electric signal via the spontaneous polarization. Benefiting from their thermal nature, pyroelectric sensors have the broad spectral response (i.e., from UV to THz). High sensitivity for pulse lasers can also be achieved in such pyroelectric sensors. Unfortunately, due to the transient response on the temperature, this type of sensors allows just measurements of pulse lasers with the response time of microseconds [5,6].
Thermoelectric sensors are subdivided into two groups, including the devices utilized the traditional longitudinal thermoelectric (LTE) effect and the ones based on the transverse thermoelectric (TTE) effect [7][8][9]. The common advantages of these thermoelectric sensors include the wide spectral response range and the high saturation threshold of laser irradiation. The standard LTE sensors are composed of electrically connected thermocouples, where the electrical and thermal flows are parallel. Due to the existence of thermal resistance layer and the size of thermoelectric legs, this type of LTE sensor gives the response time of seconds [10].
TTE sensors utilize the anisotropic Seebeck coefficients, which exhibits unique properties compared to the LTE effect. By way of example, one sole material with significant anisotropy could produce a TTE voltage [8,9,11], where at least two different materials are required to fabricate the thermocouples in LTE sensors [10]. This means much more straightforward device fabrication is available in TTE sensors with a much lower cost. In addition, the electrical and thermal flows are perpendicular to each other in TTE effect, which indicates the relatively independent management of heat and electric fluxes. For instance, an ultrafast response time of nanoseconds (ns) can be achieved by sacrificing the thickness of sensors, while the voltage signal magnitude can be maintained by elongating the length [9]. A general approach to fabricate TTE sensors is growing epitaxial thin films with anisotropic Seebeck coefficients on vicinal cut substrates to engineer a tilted angle (θ) between c-axis orientation and surface normal of thin films. When a laser irradiates on the top surface of thin film, a temperature gradient along the out-of-plane direction (on the z-axis) of thin film will be generated, and then produce a voltage in the in-plane direction (on the x-axis) due to different Seebeck coefficients between ab-plane (S ab ) and c-axis (S c ) of thin film (see Figure 1). The output in-plane voltage (U x ) induced by the TTE effect in the film surface is expressed in the following form: where ∇T z is the temperature gradient along film thickness direction, l is the irradiation length, and d is thickness of thin film [8,[12][13][14][15][16]. However, most of the previous studies on the TTE-based laser detectors just focused on the enhancement of voltage sensitivity, and less attention has been paid to the response speed. In this work, we investigate the fast response of pulse laser detectors based on the TTE effect of YBa 2 Cu 3 O 7-δ (YBCO) thin films grown on vicinal cut LaAlO 3 (001) substrates.
the broad spectral response (i.e., from UV to THz). High sensitivity for pulse lasers can also be achieved in such pyroelectric sensors. Unfortunately, due to the transient response on the temperature, this type of sensors allows just measurements of pulse lasers with the response time of microseconds [5,6]. Thermoelectric sensors are subdivided into two groups, including the devices utilized the traditional longitudinal thermoelectric (LTE) effect and the ones based on the transverse thermoelectric (TTE) effect [7][8][9]. The common advantages of these thermoelectric sensors include the wide spectral response range and the high saturation threshold of laser irradiation. The standard LTE sensors are composed of electrically connected thermocouples, where the electrical and thermal flows are parallel. Due to the existence of thermal resistance layer and the size of thermoelectric legs, this type of LTE sensor gives the response time of seconds [10].
TTE sensors utilize the anisotropic Seebeck coefficients, which exhibits unique properties compared to the LTE effect. By way of example, one sole material with significant anisotropy could produce a TTE voltage [8,9,11], where at least two different materials are required to fabricate the thermocouples in LTE sensors [10]. This means much more straightforward device fabrication is available in TTE sensors with a much lower cost. In addition, the electrical and thermal flows are perpendicular to each other in TTE effect, which indicates the relatively independent management of heat and electric fluxes. For instance, an ultrafast response time of nanoseconds (ns) can be achieved by sacrificing the thickness of sensors, while the voltage signal magnitude can be maintained by elongating the length [9]. A general approach to fabricate TTE sensors is growing epitaxial thin films with anisotropic Seebeck coefficients on vicinal cut substrates to engineer a tilted angle (θ) between c-axis orientation and surface normal of thin films. When a laser irradiates on the top surface of thin film, a temperature gradient along the out-of-plane direction (on the z-axis) of thin film will be generated, and then produce a voltage in the in-plane direction (on the x-axis) due to different Seebeck coefficients between ab-plane (Sab) and c-axis (Sc) of thin film (see Figure 1). The output in-plane voltage (Ux) induced by the TTE effect in the film surface is expressed in the following form: where ∇ is the temperature gradient along film thickness direction, l is the irradiation length, and d is thickness of thin film [8,[12][13][14][15][16]. However, most of the previous studies on the TTE-based laser detectors just focused on the enhancement of voltage sensitivity, and less attention has been paid to the response speed. In this work, we investigate the fast response of pulse laser detectors based on the TTE effect of YBa 2 Cu 3 O 7-δ (YBCO) thin films grown on vicinal cut LaAlO3 (001) substrates.

Materials and Methods
YBCO thin films (10 × 5 mm) with thickness of about 200 nm were used as the sensing layers, which were deposited on 15 • vicinal cut LaAlO 3 (LAO) (001) single crystal substrates by pulse laser deposition (PLD). The PLD system included a KrF excimer pulse laser with a wavelength of 248 nm and pulse duration of 20 ns. Laser fluence and pulse frequency were fixed at~1.8 J cm −2 and 3 Hz, respectively. A polycrystalline YBCO target was used for the ablation process. During the deposition process, the substrate temperature and flowing oxygen pressure were set at around 785-800 • C and 40-45 Pa, respectively. Then, an oxygen annealing was performed at 500 • C with the oxygen pressure of 100,000 Pa for 1 h. The phase structure of thin film was checked by an X-ray diffraction (XRD, Rigaku D/MAX 2500V/PC X). The microstructure of YBCO thin film on vicinal cut substrate was studied by a transmission electron microscopy (TEM, JEOL ARM 200F). The schematic structure of laser detectors based on the TTE effect of thin films is shown in Figure 1. Two Pt electrodes were deposited on the top surface of YBCO thin films with the irradiation length of l = 7 mm along the inclined direction. Two types of pulse lasers have been used as the irradiation sources to measure the TTE voltage response. An optical parametric oscillator (OPOTEK INC) was employed to generate pulse laser with the repetition rate of 1 Hz, the wavelength of 1000 nm, and the pulse duration of about 5-7 ns, while a solid state Q-switched laser at 1064 nm (MPL-H-1064) was utilized to produce the pulse laser with the repetition rate of 4 kHz and the pulse duration of~10 ns. The voltage signals of these TTE-based laser detectors were recorded by an oscilloscope (Rohde & Schwarz RTE 1102, 1 GHz bandwidth).

Results
The XRD pattern of θ-2θ scan of YBCO thin film grown on 15 • vicinal cut LAO (001) substrate is shown in Figure 2. Besides the diffraction peaks of LAO (001) substrate, only (00l) diffraction peaks of YBCO thin film are detected, while other orientations are not observed. This demonstrates the pure phase and the c-axis tilted growth of YBCO thin films.
YBCO thin films (10 × 5 mm) with thickness of about 200 nm were used as the s layers, which were deposited on 15° vicinal cut LaAlO3 (LAO) (001) single cryst strates by pulse laser deposition (PLD). The PLD system included a KrF excime laser with a wavelength of 248 nm and pulse duration of 20 ns. Laser fluence and frequency were fixed at ~1.8 J cm −2 and 3 Hz, respectively. A polycrystalline YBCO was used for the ablation process. During the deposition process, the substrate te ture and flowing oxygen pressure were set at around 785-800 °C and 40-45 Pa, tively. Then, an oxygen annealing was performed at 500 °C with the oxygen pres 100,000 Pa for 1 h. The phase structure of thin film was checked by an X-ray diff (XRD, Rigaku D/MAX 2500V/PC X). The microstructure of YBCO thin film on vici substrate was studied by a transmission electron microscopy (TEM, JEOL ARM 200 schematic structure of laser detectors based on the TTE effect of thin films is sh Figure 1. Two Pt electrodes were deposited on the top surface of YBCO thin films w irradiation length of l = 7 mm along the inclined direction. Two types of pulse lase been used as the irradiation sources to measure the TTE voltage response. An optic ametric oscillator (OPOTEK INC) was employed to generate pulse laser with the tion rate of 1 Hz, the wavelength of 1000 nm, and the pulse duration of about 5-7 ns a solid state Q-switched laser at 1064 nm (MPL-H-1064) was utilized to produce th laser with the repetition rate of 4 kHz and the pulse duration of ~10 ns. The voltage of these TTE-based laser detectors were recorded by an oscilloscope (Rohde & Sc RTE 1102, 1 GHz bandwidth).

Results
The XRD pattern of θ-2θ scan of YBCO thin film grown on 15° vicinal cut LA substrate is shown in Figure 2. Besides the diffraction peaks of LAO (001) substrat (00l) diffraction peaks of YBCO thin film are detected, while other orientations observed. This demonstrates the pure phase and the c-axis tilted growth of YBC films.  Figure 3a that YBCO thin film exhibits the typical layere acteristics, which is well consistent with its crystal features with the alternative st CuO2, BaO, CuO and Y layers along the c-axis [17]. Such layered structures give th Seebeck coefficient anisotropy of (Sab − Sc) = 30 µV/K [18], which is responsible origin of TTE voltage. In addition, the interface between YBCO thin film and LA strate is sharp, indicating the high quality of YBCO thin film. The fast Fourier tra The microstructure of tilted YBCO thin film has been studied by TEM. Figure 3a shows the high-resolution TEM (HRTEM) image of YBCO thin film on 15 • vicinal cut LAO (001). It is clearly seen in Figure 3a that YBCO thin film exhibits the typical layered characteristics, which is well consistent with its crystal features with the alternative stacks of CuO 2 , BaO, CuO and Y layers along the c-axis [17]. Such layered structures give the large Seebeck coefficient anisotropy of (S ab − S c ) = 30 µV/K [18], which is responsible for the origin of TTE voltage. In addition, the interface between YBCO thin film and LAO substrate is sharp, indicating the high quality of YBCO thin film. The fast Fourier transform (FFT)  The voltage response of YBCO thin film laser detector under the irradiation of pulse laser with the repetition rate of 1 Hz, the wavelength of 1000 nm, the pulse duration (τp) of 5-7 ns, and the energy of 0.5 mJ is shown in Figure 4. As seen in Figure 4, the TTE voltage of YBCO thin film exhibits the rise time (τr, 0-100% of peak value) of about 6 ns, which is much faster than other reported ones, such as 7 ns in La0.5Sr0.5CoO3 thin film , 33-100 ns in Ca3Co4O9 thin film [19], 51 ns in La0.9Ca0.1MnO3 thin film [20], and 33 ns in CuCr0.98Mg0.02O2 thin film [21]. Such a fast response speed is related with the low roomtemperature resistivity of YBCO thin film (around 5.6 × 10 −4 Ohm cm). It is believed that the low resistivity yields small optical penetration depth, and the rise time has a monotonous increasing relationship with this penetration depth [9]. In addition, the inset of Figure 4 indicates a linear relationship between peak voltage and pulse energy. The voltage response of YBCO thin film laser detector under the irradiation of pulse laser with the repetition rate of 1 Hz, the wavelength of 1000 nm, the pulse duration (τ p ) of 5-7 ns, and the energy of 0.5 mJ is shown in Figure 4. As seen in Figure 4, the TTE voltage of YBCO thin film exhibits the rise time (τ r , 0-100% of peak value) of about 6 ns, which is much faster than other reported ones, such as 7 ns in La 0.5 Sr 0.5 CoO 3 thin film, 33-100 ns in Ca 3 Co 4 O 9 thin film [19], 51 ns in La 0.9 Ca 0.1 MnO 3 thin film [20], and 33 ns in CuCr 0.98 Mg 0.02 O 2 thin film [21]. Such a fast response speed is related with the low room-temperature resistivity of YBCO thin film (around 5.6 × 10 −4 Ohm cm). It is believed that the low resistivity yields small optical penetration depth, and the rise time has a monotonous increasing relationship with this penetration depth [9]. In addition, the inset of Figure 4 indicates a linear relationship between peak voltage and pulse energy.
In addition, the decay time (τ d , 100-0% of peak value) in Figure 4 is about 42 ns, which is significantly smaller than the ones of 1000 ns in Ca 3 Co 4 O 9 thin film [19], 4000 ns in La 0.9 Ca 0.1 MnO 3 thin film [20], and more than 7000 ns in CuCr 0.98 Mg 0.02 O 2 thin film [21]. The τ d means a recovery process of the TTE sensors from a large temperature difference to a normal state without temperature difference due to the thermal diffusion. Early studies just deemed that the τ d is inversely proportional to the total thermal conductivity (k total ), with the expression of τ d ∝ d 2 c/2k total (where c is the density of thin film) [22]. More recently, a detailed analysis showed that the composition of k total is also of great value to tune the τ d , in addition to the k total [21]. Generally, k total includes electronic thermal conductivity (k e ) and lattice thermal conductivity (k l ), written as k total = k e + k l . Meanwhile, k e and k l may dominate the corresponding fast (τ f ) and slow (τ s ) components of the decay signal, respectively. Once the k e /k total is small, the decay signal may exhibit a long voltage tail down to zero, which increases the total τ d significantly. Hence, both k total and k e /k total have notable influences on τ d . The smaller τ d of 42 ns in YBCO, including the fast component τ f of 6 ns (decay 1 in Figure 4) and the slow part τ s of 28 ns (decay 2 in Figure 4), may originate from its room temperature k total of 6 W/(m·K) and k e /k total of 33.3% [23]. In the case of CuCr 0.98 Mg 0.02 O 2 thin film, its room temperature k total is as large as 8.8 W/(m·K), but its τ d remains larger than 7000 ns surprisingly [21]. The fitting on the decay time gives the fast component of 30 ns and the slow part of about 5000 ns, which may be related with its low k e /k total of 0.02% [21]. With respect to Ca 3 Co 4 O 9 , k total is just 2.4 W/(m·K), but the k e /k total is as high as 58.3% [24]. Such a large k e /k total may be responsible for its medium τ d of 1000 ns with the fast component of 124 ns and the slow part of 583 ns. Similar situations also exist in La 0.9 Ca 0.1 MnO 3 thin film with the τ d of 3500 ns, τ f of 276 ns and the τ s of about 1400 ns, which may be from its k total of 1 W/(m·K) and k e /k total of 80% [25]. The summarized details of the decay features of TTE voltage signals (including τ d , τ f , and τ s ), the thermal conductivity (k total ), and the contribution of electron thermal conductivity (k e /k total ) of these materials are shown in Table 1.  In addition, the decay time (τd, 100-0% of peak value) in Figure 4 is about which is significantly smaller than the ones of 1000 ns in Ca3Co4O9 thin film [19], 4 in La0.9Ca0.1MnO3 thin film [20], and more than 7000 ns in CuCr0.98Mg0.02O2 thin film The τd means a recovery process of the TTE sensors from a large temperature diff to a normal state without temperature difference due to the thermal diffusion. Early ies just deemed that the τd is inversely proportional to the total thermal conductivity with the expression of τd ∝ d 2 c/2ktotal (where c is the density of thin film) [22]. More re a detailed analysis showed that the composition of ktotal is also of great value to tu τd, in addition to the ktotal [21]. Generally, ktotal includes electronic thermal conductiv and lattice thermal conductivity (kl), written as ktotal = ke + kl. Meanwhile, ke and kl may inate the corresponding fast (τf) and slow (τs) components of the decay signal, r tively. Once the ke/ktotal is small, the decay signal may exhibit a long voltage tail do zero, which increases the total τd significantly. Hence, both ktotal and ke/ktotal have n influences on τd. The smaller τd of 42 ns in YBCO, including the fast component τf  Here, it is worth noting that both τ r and τ d of laser detectors should be considered for the detection of pulse lasers with repetition frequencies larger than 1 Hz, as τ d will strongly affect the measurements of the following pulses. However, most of the previous works only focus on the τ r and neglect the influences of τ d on the maximum measurable frequency of a train of pulse lasers. According to the discussion above, it is seen that the combinational properties of thin films, including the low resistivity, the high thermal conductivity and the large contribution of electronic thermal conductivity to the thermal conductivity, are beneficial to a fast and broadband response. Among the various materials, YBCO exhibits better properties comprehensively than other materials, giving rise to much faster response with smaller τ r and τ d simultaneously.
The response of YBCO thin film laser detector under the irradiation of a train of pulse lasers (τ p~1 0 ns) with a repetition rate of 4 kHz is shown in Figure 5. As seen in Figure 5, pulse signals with the interval of about 230 µs are clearly observed, which is quite close to the theoretical value of 250 µs. In addition, the amplitude of these signals shows the high precision with the standard deviation of 0.0734. These results demonstrate that the YBCObased TTE sensors have the ability to measure the pulse lasers with a high repetition rate.
frequency of a train of pulse lasers. According to the discussion above, it is combinational properties of thin films, including the low resistivity, the high ductivity and the large contribution of electronic thermal conductivity to the ductivity, are beneficial to a fast and broadband response. Among the vario YBCO exhibits better properties comprehensively than other materials, g much faster response with smaller τr and τd simultaneously.
The response of YBCO thin film laser detector under the irradiation of a lasers (τp~10 ns) with a repetition rate of 4 kHz is shown in Figure 5. As seen pulse signals with the interval of about 230 µs are clearly observed, which to the theoretical value of 250 µs. In addition, the amplitude of these signa high precision with the standard deviation of 0.0734. These results demons YBCO-based TTE sensors have the ability to measure the pulse lasers with a tion rate.

Conclusions
In summary, we have fabricated the pulse laser detectors based on th thermoelectric effect of YBa2Cu3O7-δ thin films on miscut LaAlO3 (001) substra beck coefficient anisotropy between ab-plane (Sab) and c-axis (Sc) of YBa2Cu3O is utilized to generate the output voltage via the transverse thermoelectric e trast to the traditional longitudinal thermoelectric effect. These transverse th detectors exhibit the fast response, including both the rise time and the dec diated by pulse laser with the pulse duration of 5-7 ns, the rise time and th

Conclusions
In summary, we have fabricated the pulse laser detectors based on the transverse thermoelectric effect of YBa 2 Cu 3 O 7-δ thin films on miscut LaAlO 3 (001) substrates. The Seebeck coefficient anisotropy between ab-plane (S ab ) and c-axis (S c ) of YBa 2 Cu 3 O 7-δ thin films is utilized to generate the output voltage via the transverse thermoelectric effect, in contrast to the traditional longitudinal thermoelectric effect. These transverse thermoelectric detectors exhibit the fast response, including both the rise time and the decay time. Irradiated by pulse laser with the pulse duration of 5-7 ns, the rise time and the decay time of the output voltage are 6 and 42 ns, respectively. Compared with other materials, these values of YBa 2 Cu 3 O 7-δ thin film are significantly smaller. Such a fast rise time may come from the low resistivity of YBa 2 Cu 3 O 7-δ thin films, while the small decay time may originate from its high thermal conductivity and the large contribution of electronic thermal conductivity to the thermal conductivity. In addition, these detectors are able to precisely measure the pulse lasers with a repetition rate of 4 kHz. Since this kind of laser detector based on the transverse thermoelectric effect of thin film possesses the characteristics of fast response and wide bandwidth, we believe that this work may provide a roadmap to fabricate the thin film laser detectors with high performances.